Recently developed data storage devices, such as magnetic disk drive devices (i.e., hard disk drives), have increased storage capacity and increased data access speed. The magnetic disk drive devices have become widely used as auxiliary memory devices for computer systems. The primary components of a magnetic disk drive device that affect storage capacity and access speed are the magnetic recording head, the recording medium, the servo mechanism, the signal processing technique used in the read/write channel, and the like. As real densities increase, inter-symbol interference (ISI), transition-dependent noise and non-linear distortions at high densities, and bandwidth limitations at high data rates lead to performance degradation. For example, the level of ISI between neighboring recorded bits in magnetic recording channels increases with recording density.
The signal processing techniques utilizing PRML (Partial Response Maximum Likelihood) detection have greatly contributed to the increased storage densities and high access speeds seen in modem magnetic disk drive devices. PRML data channels are synchronization data detection channels where synchronization refers to the frequency and phase locking of the channel to the readback signal in order to detect the data properly.
A Viterbi detector is used for detecting the data pulses in the digitized read signal and recovering the bits. Advanced replay equalizations have been adopted in the magnetic recording technology to shape the channel pulse response to some specified target shape, which has a shorter duration (higher bandwidth) and this is called partial-response signaling or equalization. The Viterbi detector that is matched to the target shape normally follows the partial response equalizer. The Viterbi detector recovers the encoded data that was originally recorded on the magnetic medium. Such a detector receives an equalized digital read signal and generates from it an encoded data signal, which is then decoded to produce the final read data signal.
At the heart of the Viterbi decoding algorithm is the trellis, which is an extension of the encoder state machine that shows the passage of time. A section of the trellis shows the possible state transitions and output code words for one period of the encoder. Every branch between two states represents a possible state change in the encoder. The Viterbi procedure determines the best path (most likely sequence of symbols from a finite alphabet) ending in each state j, where state j represents the memory in the channel, by comparing the samples in the sample sequence y0, y1, . . . yn to the expected sequence of read back samples associated with all possible paths that can end in state j at time n.
A 16-state equalization target is a conventional approach for providing equalization in a PRML read data channel. An improvement has been made in equalization in U.S. Pat. No. 7,286,595 to Cideciyan et al, which is incorporated by reference herein for all purposes. The improvement includes a lengthened equalization target filter with a matched filter metric in a Viterbi detector. Maintaining precisely the desired partial response shape through adaptive equalizations at the channel output, permits the Viterbi detector to be efficiently realized and thus improves the bit detection quality.
For the data channel in a disk drive to read back data written in a sector, the channel must achieve both bit synchronization and word synchronization. Bit synchronization is the process of using acquisition gain and timing loops over a preamble pattern in order to achieve proper synchronization bit sampling, that is proper gain, phase, and frequency. Word synchronization is the process of finding the exact starting location of the data after the preamble field.
In current disk drives with increasing speed and data density, conventional word synchronization schemes are insufficient and generally provide poor performance. Miss-detecting the word synchronization pattern or Synchronization Mark (“SM”) or finding it early or later, corrupts the data detection in the entire sector. When the word synchronization detector fails to start the data time-varying-trellis correctly at the start of data, then the data error rates are degraded.
An advancement has been described in U.S. Pat. No. 7,010,065 to Cideciyan et al, which is incorporated by reference herein for all purposes. There, word synchronization is used with large coding distance and fault tolerance for PRML systems.
While the above approach is useful, accurate and reliable detection of synchronization marks can be difficult to achieve owing to the large amounts of data typically stored on a disk medium. Specifically, the large volumes of data can randomly mimic the bit patterns expected from a synchronization mark, leading to a false positive detection. Accordingly, there is a need in the art for improved techniques for improving the reliability of the sync mark detection.